This invention relates generally to tightening and tightening control systems, and more particularly to systems for tightening fasteners which exhibit more than one installation region during a complete tightening cycle. Thread forming fasteners are one example of such fasteners.
In order to properly install a thread forming fastener into an unthreaded workpiece hole, a first torque value must be reached in order to form the thread and a final tightening torque must be applied in order to properly seat and tighten the fastener. These torques may be referred to, respectively, as the thread forming torque and the seating torque. In order to install a thread forming fastener, a hole of the proper size for a particular sized fastener is drilled, pierced, or extruded in the workpiece material, and the fastener is then rotated into the hole. Tolerance on the hole size is of critical importance. If the hole is too small, the torque required to drive the fastener may become so large that the fastener will fail in torsion. If the hole is too large, the integrity of the fastened joint is compromised. Workpiece material characteristics (i.e. hardness, toughness, etc.) and thickness also have an effect on the performance of a thread forming fastener. As the hardened thread of the fastener enters the hole, the fastener thread displaces the workpiece material to form a mating thread. The softer the material, the easier it is to form the threads. Conversely, if the material is hard, dense and tough, less material can be extruded and greater energy is necessary to form the thread. Thus, the required initial hole diameter for a particular size thread forming fastener depends upon a number of physical variables, all of which contribute in varying degrees to the energy or torque needed to form the thread.
Present assembly tools for installing thread forming fasteners are generally of the torque control variety. Normally, a single torque setting is selected and set into the tool, the torque value corresponding to the final desired seating value. This torque setting must be sufficiently high in order to form a mating thread under the most severe conditions of hole size, thickness, and material properties which are expected to be encountered. However, this torque must not be set so high as to cause stripping of the threads when the same variables interact to minimize the thread forming torque necessary in a particular joint. Stripping may be conveniently defined as a mode of thread failure wherein the internal thread material is sheared away from the remainder of the workpiece. Thus, the thread forming torque to stripping torque ratio becomes critical when assembling a number of joints, even in the same workpiece material. It is furthermore desirable to install a particular size fastener into a variety of holes of varying initial diameters in different materials having diverse physical characteristics with a single installation tool.
Referring to FIG. 1, the two torque vs. rotation curves shown represent extremes of physical conditions which could be encountered in two separate joints in the same or different workpieces. No single torque setting satisfies both conditions. For example, if the torque is set at a value corresponding to [(T.sub.S).sub.B ] in the installation tool, fastener B may be tightened to the correct seating torque value, but this value will not be sufficient to form the thread in fastener A. Conversely, if a torque value [(T.sub.S).sub.A ] is set in the tool, the threads in fastener B will be stripped. It is this type of problem which has severely limited the use of automatic tightening equipment for tightening thread forming fasteners, and further has limited the use of threads forming fasteners themselves in many structural applications where their use would be beneficial. These and other problems are overcome by the present invention.